Topics

Transmitter Mods

Howard Fidel
 

I got my uBitx last week. I was surprised to get it, since it was sent 3 days before and I paid for India Post shipment, but it came by DHL, which was a nice surprise. I have assembled it, and started testing and am doing a number of modifications in my lab before moving it to its home in the shack.

Today I modified the transmitter to improve the output power at the higher frequencies. The power out in the original state is about 3 watts at 28 MHz. My modification brings the power at 28 MHz up to 7 watts on both the 15 and 10 meter bands. On my board at least, there is not enough range in VR1 to increase the power to anywhere near this level without this modification. The other bands remain unchanged in output, which I measured at 11 watts.  The output stage could be modified as others have done by changing the FETs and transformer to improve it more, but I am happy with this for now. I plan on adding a power amplifier later, something between 50 and 100 watts.

My fix is simply to add 3 passive parts. I added:
33 uH inductor in series with R86. I lifted the resistor mounting it on one pad standing up, and teepeeing the inductor between the top of the resistor and the other pad.

Add a 270 pF capacitor across both R87 and R88.

The values are not very critical, I calculated that I needed 27 uH and 220 pF, but the values I tried were in the lab.

If anyone wants to try the fix and needs the parts, I have a full reel of each, so just send me a stamped self addressed envelope, and I will send it to you by return mail. The offer is good until I run out of parts.

Next up is adding an AGC and S meter.

I also measured the receiver sensitivity and can confirm that it meets specifications. 

Howard

WB2VXW


Jonathan
 

Howard,

Could you draw a little circuit diagram of your modifcation?   Thanks

-Jonathan KF6RTA

Lawrence Galea
 

Hi Howard,
Good job.
The inductor reduces the feedback at higher frequencies while the capacitors offer a lower impedance at higher frequencies across R87 and R88.
Can you try the same mod on the driver transistors Q92, Q93, Q96, Q97
Presently I don't have much time and I am very busy with other things.
Regards
Lawrence

On Tue, Apr 17, 2018 at 5:25 PM, Howard Fidel <sonic1@...> wrote:

I got my uBitx last week. I was surprised to get it, since it was sent 3 days before and I paid for India Post shipment, but it came by DHL, which was a nice surprise. I have assembled it, and started testing and am doing a number of modifications in my lab before moving it to its home in the shack.

Today I modified the transmitter to improve the output power at the higher frequencies. The power out in the original state is about 3 watts at 28 MHz. My modification brings the power at 28 MHz up to 7 watts on both the 15 and 10 meter bands. On my board at least, there is not enough range in VR1 to increase the power to anywhere near this level without this modification. The other bands remain unchanged in output, which I measured at 11 watts.  The output stage could be modified as others have done by changing the FETs and transformer to improve it more, but I am happy with this for now. I plan on adding a power amplifier later, something between 50 and 100 watts.

My fix is simply to add 3 passive parts. I added:
33 uH inductor in series with R86. I lifted the resistor mounting it on one pad standing up, and teepeeing the inductor between the top of the resistor and the other pad.

Add a 270 pF capacitor across both R87 and R88.

The values are not very critical, I calculated that I needed 27 uH and 220 pF, but the values I tried were in the lab.

If anyone wants to try the fix and needs the parts, I have a full reel of each, so just send me a stamped self addressed envelope, and I will send it to you by return mail. The offer is good until I run out of parts.

Next up is adding an AGC and S meter.

I also measured the receiver sensitivity and can confirm that it meets specifications. 

Howard

WB2VXW



Jerry Gaffke
 

Very cool!
Thanks for sharing this.
Most of us have assumed it would take significantly more modding
to bring the power up to something reasonable on the high bands.

Jerry, KE7ER



On Tue, Apr 17, 2018 at 08:26 am, Howard Fidel wrote:

Today I modified the transmitter to improve the output power at the higher frequencies. The power out in the original state is about 3 watts at 28 MHz. My modification brings the power at 28 MHz up to 7 watts on both the 15 and 10 meter bands. On my board at least, there is not enough range in VR1 to increase the power to anywhere near this level without this modification. The other bands remain unchanged in output, which I measured at 11 watts.  The output stage could be modified as others have done by changing the FETs and transformer to improve it more, but I am happy with this for now. I plan on adding a power amplifier later, something between 50 and 100 watts.

My fix is simply to add 3 passive parts. I added:
33 uH inductor in series with R86. I lifted the resistor mounting it on one pad standing up, and teepeeing the inductor between the top of the resistor and the other pad.

Add a 270 pF capacitor across both R87 and R88.

The values are not very critical, I calculated that I needed 27 uH and 220 pF, but the values I tried were in the lab.

 

Howard Fidel
 

Yes, I know. I just didn't get around to it yet. Twice as many parts to solder.

On 4/17/2018 5:16 PM, Lawrence Galea wrote:
Hi Howard,
Good job.
The inductor reduces the feedback at higher frequencies while the capacitors offer a lower impedance at higher frequencies across R87 and R88.
Can you try the same mod on the driver transistors Q92, Q93, Q96, Q97
Presently I don't have much time and I am very busy with other things.
Regards
Lawrence

On Tue, Apr 17, 2018 at 5:25 PM, Howard Fidel <sonic1@...> wrote:

I got my uBitx last week. I was surprised to get it, since it was sent 3 days before and I paid for India Post shipment, but it came by DHL, which was a nice surprise. I have assembled it, and started testing and am doing a number of modifications in my lab before moving it to its home in the shack.

Today I modified the transmitter to improve the output power at the higher frequencies. The power out in the original state is about 3 watts at 28 MHz. My modification brings the power at 28 MHz up to 7 watts on both the 15 and 10 meter bands. On my board at least, there is not enough range in VR1 to increase the power to anywhere near this level without this modification. The other bands remain unchanged in output, which I measured at 11 watts.  The output stage could be modified as others have done by changing the FETs and transformer to improve it more, but I am happy with this for now. I plan on adding a power amplifier later, something between 50 and 100 watts.

My fix is simply to add 3 passive parts. I added:
33 uH inductor in series with R86. I lifted the resistor mounting it on one pad standing up, and teepeeing the inductor between the top of the resistor and the other pad.

Add a 270 pF capacitor across both R87 and R88.

The values are not very critical, I calculated that I needed 27 uH and 220 pF, but the values I tried were in the lab.

If anyone wants to try the fix and needs the parts, I have a full reel of each, so just send me a stamped self addressed envelope, and I will send it to you by return mail. The offer is good until I run out of parts.

Next up is adding an AGC and S meter.

I also measured the receiver sensitivity and can confirm that it meets specifications. 

Howard

WB2VXW




MVS Sarma
 

inductor is likely to short the resistor if connected in parallel.
 I suppose that the concerned resistors have to be lifted and inductor to be in series to it. or have inductor inseries with a 0.1uF in series to connect that assembly parallel to the said resistors.
regards
 sarma
 vu3zmv

Regards
MVS Sarma
 

On Wed, Apr 18, 2018 at 2:46 AM, Lawrence Galea <9h1avlaw@...> wrote:
Hi Howard,
Good job.
The inductor reduces the feedback at higher frequencies while the capacitors offer a lower impedance at higher frequencies across R87 and R88.
Can you try the same mod on the driver transistors Q92, Q93, Q96, Q97
Presently I don't have much time and I am very busy with other things.
Regards
Lawrence

On Tue, Apr 17, 2018 at 5:25 PM, Howard Fidel <sonic1@...> wrote:

I got my uBitx last week. I was surprised to get it, since it was sent 3 days before and I paid for India Post shipment, but it came by DHL, which was a nice surprise. I have assembled it, and started testing and am doing a number of modifications in my lab before moving it to its home in the shack.

Today I modified the transmitter to improve the output power at the higher frequencies. The power out in the original state is about 3 watts at 28 MHz. My modification brings the power at 28 MHz up to 7 watts on both the 15 and 10 meter bands. On my board at least, there is not enough range in VR1 to increase the power to anywhere near this level without this modification. The other bands remain unchanged in output, which I measured at 11 watts.  The output stage could be modified as others have done by changing the FETs and transformer to improve it more, but I am happy with this for now. I plan on adding a power amplifier later, something between 50 and 100 watts.

My fix is simply to add 3 passive parts. I added:
33 uH inductor in series with R86. I lifted the resistor mounting it on one pad standing up, and teepeeing the inductor between the top of the resistor and the other pad.

Add a 270 pF capacitor across both R87 and R88.

The values are not very critical, I calculated that I needed 27 uH and 220 pF, but the values I tried were in the lab.

If anyone wants to try the fix and needs the parts, I have a full reel of each, so just send me a stamped self addressed envelope, and I will send it to you by return mail. The offer is good until I run out of parts.

Next up is adding an AGC and S meter.

I also measured the receiver sensitivity and can confirm that it meets specifications. 

Howard

WB2VXW




Jerry Gaffke
 

The inductor is not in parallel with a resistor,
Howard recommends placing it in series with a resistor:


On Tue, Apr 17, 2018 at 5:25 PM, Howard Fidel <sonic1@...> wrote:
>  33 uH inductor in series with R86. I lifted the resistor mounting it on one pad standing up,
>  and teepeeing the inductor between the top of the resistor and the other pad.
>  Add a 270 pF capacitor across both R87 and R88. 

The two caps he added are in parallel with resistors.

Jerry, KE7ER


On Tue, Apr 17, 2018 at 09:16 pm, Mvs Sarma wrote:
inductor is likely to short the resistor if connected in parallel.
 I suppose that the concerned resistors have to be lifted and inductor to be in series to it. or have inductor inseries with a 0.1uF in series to connect that assembly parallel to the said resistors.
regards
 sarma
 vu3zmv

K9HZ <bill@...>
 

I read: “33 uH inductor in series with R86”

 

 

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Rent it: www.VillaGrandPiton.com

Like us on Facebook! facebook icon

 

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From: BITX20@groups.io [mailto:BITX20@groups.io] On Behalf Of Mvs Sarma
Sent: Tuesday, April 17, 2018 11:16 PM
To: BITX20@groups.io
Subject: Re: [BITX20] Transmitter Mods

 

inductor is likely to short the resistor if connected in parallel.

 I suppose that the concerned resistors have to be lifted and inductor to be in series to it. or have inductor inseries with a 0.1uF in series to connect that assembly parallel to the said resistors.

regards

 sarma

 vu3zmv


Regards
MVS Sarma
 

 

On Wed, Apr 18, 2018 at 2:46 AM, Lawrence Galea <9h1avlaw@...> wrote:

Hi Howard,

Good job.

The inductor reduces the feedback at higher frequencies while the capacitors offer a lower impedance at higher frequencies across R87 and R88.

Can you try the same mod on the driver transistors Q92, Q93, Q96, Q97

Presently I don't have much time and I am very busy with other things.

Regards

Lawrence

 

On Tue, Apr 17, 2018 at 5:25 PM, Howard Fidel <sonic1@...> wrote:

I got my uBitx last week. I was surprised to get it, since it was sent 3 days before and I paid for India Post shipment, but it came by DHL, which was a nice surprise. I have assembled it, and started testing and am doing a number of modifications in my lab before moving it to its home in the shack.

Today I modified the transmitter to improve the output power at the higher frequencies. The power out in the original state is about 3 watts at 28 MHz. My modification brings the power at 28 MHz up to 7 watts on both the 15 and 10 meter bands. On my board at least, there is not enough range in VR1 to increase the power to anywhere near this level without this modification. The other bands remain unchanged in output, which I measured at 11 watts.  The output stage could be modified as others have done by changing the FETs and transformer to improve it more, but I am happy with this for now. I plan on adding a power amplifier later, something between 50 and 100 watts.

My fix is simply to add 3 passive parts. I added:
33 uH inductor in series with R86. I lifted the resistor mounting it on one pad standing up, and teepeeing the inductor between the top of the resistor and the other pad.

Add a 270 pF capacitor across both R87 and R88.

The values are not very critical, I calculated that I needed 27 uH and 220 pF, but the values I tried were in the lab.

If anyone wants to try the fix and needs the parts, I have a full reel of each, so just send me a stamped self addressed envelope, and I will send it to you by return mail. The offer is good until I run out of parts.

Next up is adding an AGC and S meter.

I also measured the receiver sensitivity and can confirm that it meets specifications. 

Howard

WB2VXW

 

 

 


Virus-free. www.avg.com

Howard Fidel
 

If you read the instructions, it is clear the inductor is in series with the resistor and provides high frequency peaking.

Howard


On 4/18/2018 12:16 AM, Mvs Sarma wrote:
inductor is likely to short the resistor if connected in parallel.
 I suppose that the concerned resistors have to be lifted and inductor to be in series to it. or have inductor inseries with a 0.1uF in series to connect that assembly parallel to the said resistors.
regards
 sarma
 vu3zmv

Regards
MVS Sarma
 

On Wed, Apr 18, 2018 at 2:46 AM, Lawrence Galea <9h1avlaw@...> wrote:
Hi Howard,
Good job.
The inductor reduces the feedback at higher frequencies while the capacitors offer a lower impedance at higher frequencies across R87 and R88.
Can you try the same mod on the driver transistors Q92, Q93, Q96, Q97
Presently I don't have much time and I am very busy with other things.
Regards
Lawrence

On Tue, Apr 17, 2018 at 5:25 PM, Howard Fidel <sonic1@...> wrote:

I got my uBitx last week. I was surprised to get it, since it was sent 3 days before and I paid for India Post shipment, but it came by DHL, which was a nice surprise. I have assembled it, and started testing and am doing a number of modifications in my lab before moving it to its home in the shack.

Today I modified the transmitter to improve the output power at the higher frequencies. The power out in the original state is about 3 watts at 28 MHz. My modification brings the power at 28 MHz up to 7 watts on both the 15 and 10 meter bands. On my board at least, there is not enough range in VR1 to increase the power to anywhere near this level without this modification. The other bands remain unchanged in output, which I measured at 11 watts.  The output stage could be modified as others have done by changing the FETs and transformer to improve it more, but I am happy with this for now. I plan on adding a power amplifier later, something between 50 and 100 watts.

My fix is simply to add 3 passive parts. I added:
33 uH inductor in series with R86. I lifted the resistor mounting it on one pad standing up, and teepeeing the inductor between the top of the resistor and the other pad.

Add a 270 pF capacitor across both R87 and R88.

The values are not very critical, I calculated that I needed 27 uH and 220 pF, but the values I tried were in the lab.

If anyone wants to try the fix and needs the parts, I have a full reel of each, so just send me a stamped self addressed envelope, and I will send it to you by return mail. The offer is good until I run out of parts.

Next up is adding an AGC and S meter.

I also measured the receiver sensitivity and can confirm that it meets specifications. 

Howard

WB2VXW





 

Thanks Howard for this simple mod. I did the mod this morning with spectacular results. Before the mod I had only 300mw at 13V on 28.500, after the mod I had 3.8 watts and after increasing the drive slightly I now have 4.5 watts. All of the other bands benefited from it also. All in all, a great mod. Thanks!

Joel
N6ALT

ohwenzelph
 

How and why does this mod work? What was the calculation that came up with 220pF and 27 uH? 
For the inductor does it matter how many ma it can handle or what it’s self resonant frequency is?
(doubt I will be able to follow the answer but still curious)
thanks!

Howard Fidel
 

The theory is very simple. The closed loop gain of Q911 and Q912 is set by the ratio of R86/R85, or a gain of 10. Adding an inductor into the path in the feedback loop increases the effective impedance at higher frequencies, adding "peaking" to the circuit, thus making the closed loop gain higher. I think the poor frequency response is in the transformer as well as the transistors hfe falling to under 25 typically at 30 MHz. Also, the open loop gain is also increased by bypassing the emitter resistors for higher frequencies.

The transformer is a 2 to 1 step down, (it looks like a trifilar winding) so the theoretical gain of the stage is 5.

Howard

On 4/19/2018 12:36 AM, ohwenzelph via Groups.Io wrote:
How and why does this mod work? What was the calculation that came up with 220pF and 27 uH? 
For the inductor does it matter how many ma it can handle or what it’s self resonant frequency is?
(doubt I will be able to follow the answer but still curious)
thanks!


Gordon Gibby <ggibby@...>
 

howard -- thank you for that review!   The inductor I find is up into the 2-3K  (ohms reactance) range by 20 MHz so it definitely changes the feedback ratio at the higher frequencies.  Capacitors across the emitter resistor likewise reduce the negative feedback at higher F.    This little improvement is one of the coolest things I've read yet about the uBitx power issues --- and also the idea of adding C across the power amplifier output transformer to better match --- 



I'm encouraging our Local ARES group to move into these rigs as a learning platform and also for some, their first HF rig -- can't afford the pricey ones --- wonderful platform for LEARNING.     Getting 10W or so on 80/40/30 would be great for NVIS emergency communications for many of these people.   At at a far far cheaper price than commercial fancy qrp rigs --- so next is to put one of mine in an ammo can for protection.


gordon




From: BITX20@groups.io <BITX20@groups.io> on behalf of Howard Fidel <sonic1@...>
Sent: Thursday, April 19, 2018 10:00 AM
To: BITX20@groups.io
Subject: Re: [BITX20] Transmitter Mods
 
The theory is very simple. The closed loop gain of Q911 and Q912 is set by the ratio of R86/R85, or a gain of 10. Adding an inductor into the path in the feedback loop increases the effective impedance at higher frequencies, adding "peaking" to the circuit, thus making the closed loop gain higher. I think the poor frequency response is in the transformer as well as the transistors hfe falling to under 25 typically at 30 MHz. Also, the open loop gain is also increased by bypassing the emitter resistors for higher frequencies.

The transformer is a 2 to 1 step down, (it looks like a trifilar winding) so the theoretical gain of the stage is 5.

Howard

On 4/19/2018 12:36 AM, ohwenzelph via Groups.Io wrote:
How and why does this mod work? What was the calculation that came up with 220pF and 27 uH? 
For the inductor does it matter how many ma it can handle or what it’s self resonant frequency is?
(doubt I will be able to follow the answer but still curious)
thanks!


ohwenzelph
 

does it matter that most 27uH inductors seem to have a self resonant frequency below 28MHz?

Gordon Gibby <ggibby@...>
 

​I found some on digikey with a self resonaance 40 Mhz or higher....


probably does matter.   At the self resonant point the series impeane will be very low....




From: BITX20@groups.io <BITX20@groups.io> on behalf of ohwenzelph via Groups.Io <Ohwenzelph@...>
Sent: Thursday, April 19, 2018 12:59 PM
To: bitx20@groups.io
Subject: Re: [BITX20] Transmitter Mods
 
does it matter that most 27uH inductors seem to have a self resonant frequency below 28MHz?

Howard Fidel
 

That is a good question. In our application, the impedance is still high, it just becomes more capacitive then inductive. If you look at the impedance plot, the resonance is about 20 MHz, but the reactance at 30 MHz is still 2000 ohms, so the gain goes up. On the 15 meter band, it looks like 30K ohms, so the amp runs open loop.
The bottom line is that the design is not that robust to begin with, and what I have made is a patch. I would never have designed the signal path myself this way. I would have use an opamp to drive the transformer to replace everything from C80 to T10.
The fix works, and has been validated by others. It would be smaller, probably not cheaper, but robust.

Howard

On 4/19/2018 12:59 PM, ohwenzelph via Groups.Io wrote:
does it matter that most 27uH inductors seem to have a self resonant frequency below 28MHz?


Howard Fidel
 

Gordon:
Very high, about 30 K. It actually helps us.

Howard

On 4/19/2018 1:03 PM, Gordon Gibby wrote:

​I found some on digikey with a self resonaance 40 Mhz or higher....


probably does matter.   At the self resonant point the series impeane will be very low....




From: BITX20@groups.io <BITX20@groups.io> on behalf of ohwenzelph via Groups.Io <Ohwenzelph@...>
Sent: Thursday, April 19, 2018 12:59 PM
To: bitx20@groups.io
Subject: Re: [BITX20] Transmitter Mods
 
does it matter that most 27uH inductors seem to have a self resonant frequency below 28MHz?


Gordon Gibby <ggibby@...>
 

How is a series resonance 30,000 ohms?   

I’m kind of dumb, but I thought at series resonance , reactive components cancel and all that is left is the tiny ohmic resistance of the wire. 

What am I missing?

Thanks

Gordon



On Apr 19, 2018, at 13:22, Howard Fidel <sonic1@...> wrote:

Gordon:
Very high, about 30 K. It actually helps us.

Howard

On 4/19/2018 1:03 PM, Gordon Gibby wrote:

​I found some on digikey with a self resonaance 40 Mhz or higher....


probably does matter.   At the self resonant point the series impeane will be very low....




From: BITX20@groups.io <BITX20@groups.io> on behalf of ohwenzelph via Groups.Io <Ohwenzelph@...>
Sent: Thursday, April 19, 2018 12:59 PM
To: bitx20@groups.io
Subject: Re: [BITX20] Transmitter Mods
 
does it matter that most 27uH inductors seem to have a self resonant frequency below 28MHz?


Jerry Gaffke
 

Good question.
It's a parallel resonance:  
    https://www.everythingrf.com/community/what-is-self-resonant-frequency



On Thu, Apr 19, 2018 at 10:55 am, Gordon Gibby wrote:
How is a series resonance 30,000 ohms?   
 
I’m kind of dumb, but I thought at series resonance , reactive components cancel and all that is left is the tiny ohmic resistance of the wire. 
 
What am I missing?
 

Gordon Gibby <ggibby@...>
 

Aha!!! 

Thanks.  Didn’t realize it was parallel. 

Much appreciated.  Learn something every day!

Gordon




On Apr 19, 2018, at 14:00, Jerry Gaffke via Groups.Io <jgaffke@...> wrote:

Good question.
It's a parallel resonance:  
    https://www.everythingrf.com/community/what-is-self-resonant-frequency



On Thu, Apr 19, 2018 at 10:55 am, Gordon Gibby wrote:
How is a series resonance 30,000 ohms?   
 
I’m kind of dumb, but I thought at series resonance , reactive components cancel and all that is left is the tiny ohmic resistance of the wire. 
 
What am I missing?